Alternating electric current rectifier of the selenium type



Patented Apr. 7, 1942 ALTERNATING ELECTRIC CURRENT RECTI- FIER OF THE SELENIUM TYPE- Lcslie Ernest Thompson and Alexander Jenkins,

London, England, assignors, by mesne assignments, to The Union Switch & Signal Company, Swissvale, Pa.,-a corporation of Pennsylvania No Drawing. Application December 13, 1939, Serial No. 309,040. In Great Britain January 7 Claims. (Cl. 175-466) This invention relates to alternating electric current rectifiers of the selenium type comprising a base plate or support provided with a layer o coating of selenium which may or may not contain other substances or compounds of selenium, a counterelectrode being applied to the surface of the selenium layer;

According to the invention, after the seleniu coating or layer on the base plate or support has been subjected to an annealing or heat treatment for the purpose of effecting orcompleting the conversion of the selenium into a metallic electrically conducting state, a counterelectrode composed preferably of tin, cadmium or alloys of these metals is applied for instance by a spraying or sputtering operation to the surface of the selenium, and a voltage which is high relative to the normal voltage at which the rectifier operates is applied to the rectifier in the reverse di rection with the result that the reverse resistance of the rectifier is very considerably increased without the. necessity for any chemical treatment.

It is believed that thisaction is due to local heating at the-boundary surface between the base plate and the selenium layer or at the boundary surface between the selenium layer and the counterelectrode or at both these surfaces.

In carrying the invention into practice it is necessary to avoid oxidation of the counterelectrode which is liable to occur due to the relatively high temperature attained by the counterelectrode if the application of the relatively high voltage is maintained for any considerable period of time. This oxidation would cause the forward resistance of the rectifier to be greatly increased and according to a further feature of the invention this disadvantage is overcome by applying the relatively high voltage in successive stages or cycles, each comprising an application of the voltage for a relatively short period followed by a period in which the rectifier is allowed to cool or is positively cooled by an air blast for example.

The high temperature required for the forming operation is thus attained without the duration of this temperature being sufficient to cause appreciable oxidation of the counterelectrode.

In order that the nature of the invention may be clearly understood a preferred process embodying the invention will now be briefly described by way of example.

A selenium rectifier produced by applying a layer or coating of selenium to a base plate or support and annealed by heat treatment to convert the selenium into the metallic conducting state is, Without any previous chemical treatment,

provided with a counterelectrode by spraying an alloy of cadmium and tin on to the surface of the selenium layer so as to produce a relatively thin metallic coating.

At a normal voltage of 6 volts impressed upon the rectifier in the reverse direction, the current traversing the rectifier is under these conditions of the order of 10 m. a. per sq. cm.

The applied reverse voltage is now increasedto approximately 15 volts whereupon the current traversing the rectifier is increased to approximately 200 m. a. per sq. cm. and the rectifier temperature rises to over 100 C.

The applied reverse voltage is now reduced to zero and the rectifier is allowed to cool to approximately room temperature, whereupon the applied voltage is again increased until the current traversing the rectifier is approximately 200 m. a. and a reverse voltage of approximately 20 volts will be found necessary for this purpose, and the temperature of the rectifier will again rise to over 100 C. a

After the rectifier has been again allowed to cool the reverse current traversing the rectifier for an applied voltage of 12 volts will generally be found to be approximately 2 m. a. per sq. cm. but if the current under these conditions is found to be appreciably higher the rectifier should be again subjected to the forming cycle above described. In general it will be found that two or three such cycles are sufficient to reduce the reverse current to a satisfactorily low value.

The resistance of the rectifier in the forward direction is found to be somewhat increased after the forming process but the final ratio of the reverse and forward resistance is such as to give commercially satisfactory rectification.

While, as above described, the temperature of the rectifier is found to rise to over 100 C. after the application of the reverse voltage, it is probable that at one or both of the boundary surfaces above referred to between the selenium. and the base plate and counterelectrode, localised heating occurs which causes the selenium to melt at the boundary surface and thereby be transformed into an amorphous or vitreous state. A thin film of selenium in this condition may thus be formed at one or both of the boundary surfaces constituting a barrier layer which co-operates with the base plate or counterelectrode to effect the observed high degree of rectification eventually obtained.

It should be noted that the counterelectrode employed preferably consists of the metals or alloys above specified and the all ys of these metals having a relatively high melting point are preferable in order to avoid their oxidation under the temperatures attained as far as possible. Counterelectrodes composed of certain other metals or alloys may however be employed and it is obviously preferable to utilise metals or alloys which do not melt or oxidise at a relatively low temperature.

It has been found however that a counterelectrode composed of zinc, gold or carbon is quite unsuitable and does not enable the advantageous ing or layer has been subjected to a heat treat-.

ment and a counterelectrode of tin, cadmium, or an alloy' of these metals is applied thereto, a voltage which is high relative to the normal voltage at which the rectifier operates is applied to the' rectifier in the reverse direction in successive stages or cycles each comprising a relatively short period of voltage application followed by a period in which the rectifier is cooled.

2. A process for the manufacture of alternating electric current rectifiers of the selenium type in which, after the selenium coating or layer has been subjected to a heat treatment and a counterelectrode applied thereto, an electromotive force of approximately volts is first applied to the rectifier in the reverse direction to heat it to a temperature of over 100 C., the applied electromotive force is then removed and the rectifier is allowed to cool, and an electromotive force of approximately volts is then applied to the rectifier in the reverse direction.

3. A process for the manufacture of alternating electric current rectifiers of the selenium type in which. after the selenium coating or layer has been subjected to a heat treatment and a counterper sq. cm. is again caused to flow through the rectifier for a period of time sufiicient to heat the rectifier to a temperature of over C.

4. A process as claimed in claim 2 in which the two last mentioned heating steps separated by a cooling step are repeated if the reverse current is higher than 2 m. a. per sq.-cm. for an applied voltage of 12 volts after the first application to the rectifier of these steps.

5. A process as claimed in claim 3 in which the two last mentioned heating steps separated by a cooling step are repeated if the reverse current is higher than 2 m. a. per sq. cm. [or an applied voltage of 12 volts alter the first application to the rectifier of these steps.

6. A process for the manufacture of alternating electric current rectifiers of-the selenium type in which after the selenium coating or layer has been subjected to a heat treatment and acounterelectrode applied thereto, a voltage which is high relative to the normal voltage at which the rectifier operates is arranged to be applied to' the rectifier in the reverse direction in successive stages or cycles each comprising a relatively short period of voltage application followed by a period in which the rectifier is cooled, for the purpose specified.

7. A process for the manufacture of alternating electric current rectifiers of the selenium type in which after the selenium coating or layer has been subjected to a heat treatment and a counterelectrode composed of tin, cadmium or an alloy of these metals applied thereto, a voltage which is high relative to the normal voltage at which the rectifier operates is applied to the rectifier in the reverse direction, for the purpose specified.

LESLIE ERNEST THOMPSON. ALEXANDER JENKINS.

DISCLAIM-ER 2,2 79,187.vLeslie Ernest Thompson and Alexander Jenkins, London, England. ALTERNAT NG ELECTRIC CURRENT RECTIFIER OF THE SELENIUM 'TYPE.

Patent dated April 7, 1942. Disclaimer filed April 25, 1942, by the assignee,

The Union Switch and Signal Company. Hereby disclaims the subject matter of claims 1, 6, and 7.

[Oflicz'al Gazette May 26, 1942.] 

